A recent desired electrophotographic image forming apparatus using toner, such as a printer, copier, and facsimile machine, is the one that can form an image on not only plain paper but also coated paper, which is glossier than plain paper, similarly to the plain paper. Therefore, for current image forming apparatuses, more and more products are becoming able to form an image on coated paper with gloss suited for the coated paper. Generally, fixing toner on coated paper uses a mechanism for obtaining high gloss by flattening a toner layer more reliably to level a toner image on the surface of the coated paper, in comparison with fixing a toner image on plain paper.
There are various types of glossy paper, such as ordinary coated paper and thermoplastic resin coated paper having a toner reception layer to be fused in fixing. For ordinary coated paper, its coating material is not fused at a fixing temperature, and thus toner fused in a fixing process is diffused along the direction of the surface of the coated paper. In contrast, for thermoplastic resin coated paper, its toner reception layer is fused at a fixing temperature, and thus part of the toner fused in a fixing process is fixed so as to be embedded in the toner reception layer. Accordingly, there is a problem in that the density of an image is not uniform because a way of fixing toner is different depending on the type of coated paper. That is, even if toner having the same area and the same amount is placed, an area covered by the toner for ordinary coated paper is larger than that for thermoplastic resin coated paper because of the difference of a fixing process. Therefore, even if an image is formed under the same conditions, the density of the image on ordinary coated paper is higher than that on thermoplastic resin coated paper.
To address this, in the case of image formation on coated paper, a method of setting an image forming condition suited for the type of coated paper with the aim of making the density of the image quality uniform is proposed (see Patent Literature 1). Patent Literature 1 discloses a method of making the screen line number in image formation on ordinary coated paper smaller than that in image formation on thermoplastic resin coated paper.